1. Field of the Invention
The present invention relates to a throttle device for engines, and more particularly to a throttle device which has a rotation position sensor for detecting an opening angle of a throttle valve.
2. Related Art
A conventional throttle device used for automotive engines has a throttle valve disposed in a throttle body forming an intake air passage. The throttle valve is fixed to a throttle shaft to rotate therewith for varying an opening area of the intake air passage, i.e., the amount of intake air supplied to the engine. In case the throttle valve is driven electrically by a motor, a rotation position sensor is used to detect an actual rotational position of the throttle valve for a throttle feedback control.
JP-A 6-117802 discloses a throttle device having a rotation position sensor. This sensor comprises a movable contact member fixed to a throttle shaft and a fixed resistor member held stationary relative to the throttle shaft, and are so arranged that the contact member driven by a throttle shaft slides over the resistor member in the circumferential direction to produce an electric voltage signal indicative of a throttle rotation position.
In the above throttle device, the throttle body is made of a material such as aluminum or resin for reducing weight, while the throttle shaft is made of such a material as iron for maintaining rigidity. The throttle device is used in the engine compartment and subjected to a large temperature change, e.g., heating and cooling of the engine and the throttle driving motor. The throttle shaft expands and contracts relative to the throttle body, because of the difference in the thermal expansion coefficients between the throttle body and the throttle shaft. The contact member of the rotation position sensor may thus change its position in the circumferential direction, sliding to a different position on the resistor member. This temperature-dependent position change causes the sensor to produce different detection outputs for the same rotation position of the throttle valve.
The throttle device may be used for an engine idle speed control, in which the throttle valve is maintained variably at a position close to the throttle full closure position. The above erroneous output from the sensor, particularly a sensor output indicating a larger throttle opening will angle than the actual opening, continue to drive the motor in the throttle closing direction even when the throttle valve is already at the full closure position. Thus, an excessive electric current continues to flow in a direction to close the throttle valve and damages the motor.